1. Field of the Invention
The invention relates to the purification of gas streams. More particularly, it relates to a pressure swing adsorption air prepurifier for the removal of contaminants from feed air prior to cryogenic separation thereof.
2. Description of the Prior Art
The removal of the contaminants present in feed air is a necessary requirement prior to the cryogenic separation of said feed air into its primary constituents, i.e. nitrogen, oxygen, argon and the like. The primary contaminants of air that need to be removed are water, carbon dioxide and hydrocarbons. Since water and carbon dioxide have much higher boiling points than the primary constituents of air, they can condense out and foul the surfaces of heat exchange passages within cryogenic air separation equipment. Hydrocarbons, e.g. acetylene, if not removed from the feed air stream entering the cryogenic air separation plant, can accumulate within said cryogenic plant and can create, in the presence of oxygen, an explosive potential within the cryogenic plant.
The use of adsorbent materials to adsorb water, carbon dioxide and hydrocarbons has been found to be possible when the contaminants are present at high pressure and at moderate temperatures. The pressure swing adsorption (PSA) process can be employed to facilitate the use of such adsorbent materials to selectively adsorb contaminants from a feed air stream at high pressure until the adsorbent materials are very nearly saturated with such contaminants. The contaminants are then desorbed from the adsorbent materials by purging the adsorbent materials with a contaminant-free gas stream at low pressure. This contaminant-free gas stream is typically the waste stream from a cryogenic plant containing primarily oxygen and nitrogen in varying amounts. During the adsorption portion of this adsorption/desorption process, the water present in feed air is typically selectively adsorbed most strongly by the adsorbent materials, with carbon dioxide being the next contaminant in feed air most strongly selectively adsorbed, with the hydrocarbon contaminant of said feed air being selectively adsorbed less strongly than the other contaminants.
The capacity of the adsorbent materials to adsorb contaminants from feed air is adversely affected by relatively high temperature adsorption conditions. In this regard, it is noted that the air temperature entering the PSA air prepurifier can be 10-15.degree. F. higher than ambient temperature due to the heat of compression of the feed air, and the use of moderate temperature cooling water obtained from air cooling tower heat exchangers to cool the compressed feed air. When the water present in the compressed air is adsorbed by the adsorbent material in a PSA system, heat is generated, released and transferred to the feed air stream, raising its temperature and that of the surrounding adsorbent material. Thus, the capacity of the adsorbent material to adsorb carbon dioxide and hydrocarbons, as well as additional quantities of water, is adversely affected by the initial adsorption of water. Conversely, when water is desorbed from the adsorbent material, heat is removed from the low pressure purge gas stream, thereby cooling the purge gas stream and reducing its effectiveness with respect to the desorption of contaminants from the adsorbent material.
PSA air prepurifiers typically employ two or more adsorption vessels so that the flow of decontaminated feed air to the cryogenic air separation plant can remain relatively steady. In order to compensate for such reduced adsorption capacity of adsorbent materials at higher temperatures, an acceptance of the reduced operating capacity of the adsorbent materials is commonly made. This however, results in undesirably increased operating costs due to the need for larger amounts of adsorbent material and an increase in the size of the adsorption vessels therefor. This could also result in higher temperature air entering downstream equipment with possibly detrimental effects. In another approach, external heat exchange devices can be used, with cool purge gas exiting from an adsorption vessel during the desorption portion of the operation in that vessel being used to cool the inlet feed air stream to another adsorption vessel than the one being used to adsorb contaminants from the feed air stream. However, this approach also requires additional capital equipment that necessarily increases the overall cost of the adsorption system.
Other means for removing contaminants from a feed air stream entering a cryogenic air separation plant include the use of thermal swing adsorption (TSA) systems, which use both temperature and pressure as driving forces to cause alternate adsorption and desorption of the contaminants. Reversing heat exchangers can also be used to freeze the water and carbon dioxide contaminants, causing them to plate out on the heat exchanger surfaces. The alternating flow into and out of the heat exchanger in a cryogenic plant allows continuous operation as the flow out of the cryogenic plant will desorb and regasify the feed air contaminants and purge them to the atmosphere.
Such other means will be seen to likewise necessitate relatively high capital and/or operating costs to achieve the desired removal of the above-indicated contaminants from the feed air to a cryogenic air separation plant. There is a desire, in the art, therefore, to develop an air prepurifier capable of more economically removing the contaminants of feed air being passed to a cryogenic air separation plant. There is also a desire to likewise develop economical prepurifiers for use in the purification of other gas streams.
It is an object of the invention to provide a prepurifier capable of economically removing the contaminants from a feed gas stream.
It is another object of the invention to provide an air prepurifier capable of economically removing the contaminants from feed air passing to a cryogenic air separation plant.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.